    (a) This invention relates to a collector made of a polyolefinic fiber having an amidoxime group and a hydrophilic group and which is capable of efficient adsorptive recovery of useful metals such as uranium, vanadium, cobalt and titanium that occur dissolved in small quantities in seawater. The invention also relates to a process for producing the collector.
Seawater has various metals (see Table 1) dissolved in it and the present invention aims at recovering these dissolved metals by adsorption using a collector.
TABLE 1Total estimatedConcentrationdissolvedDependencyRare metalin seawater,quantity,on overseas,sources(mg/ton)(×108 tons)(%)Cobalt (Co)0.11100Yttrium (Y)0.33100Titanium (Ti)115100Manganese (Mn)23090Vanadium (V)230100Uranium (U)345100Molybdenum (Mo)10150100Lithium (Li)1702,330100Boron (B)4,60063,020100Strontium (Sr)8,000109,600100(b) The invention relates to a collector that is produced by introducing an amidoxime group, either alone or in combination with a hydrophilic group, into side chains grafted to a polyolefinic fiber substrate and which needs only to be anchored in seawater to accomplish efficient recovery of useful metals such as vanadium, cobalt, uranium and titanium that are dissolved in the seawater. The invention also relates to a cassette of such collectors and a method of collecting the above-mentioned useful metals from seawater using the cassette.
To produce the collector of the invention, a polymerizable monomer such as acrylonitrile (CH2═CHCN) that contains a cyan group (—CN) is grafted onto a polyolefinic fiber substrate by radiation-initiated graft polymerization so as to form grafted side chains and the cyan groups in these side chains are reacted with hydroxylamine (NH2OH) or the like to be converted to amidoxime groups.
A plurality of the thus produced collectors may be sandwiched between nets and a plurality of the resulting assemblies are stacked in position at suitable spacings to construct a collector cassette. The cassette may be placed in a number of cages that are anchored in seawater to recover useful dissolved metals from it by adsorption.(a) Conventionally, amidoxime groups are introduced into a polymer structure in accordance with the following scheme (1) by reacting the cyano group (—CN) with hydroxylamine (NH2OH): 
To synthesize a satisfactory amidoxime resin by introducing amidoxime groups into a polymer structure, the introduction of amidoxime groups into substrates typically made of the general-purpose polyacrylic fiber or polyacrylic beads produced by emulsion polymerization. However, these acrylic resins have suffered from deterioration in skeletal strength of the polymer on account of the introduction of hydrophilic amidoxime groups into the cyano groups in the polymer skeleton. With a view to preventing this problem, a review has been made to form crosslinks in the polymer structure. In fact, however, the increase in the degree of crosslinking is accompanied by a decrease in the rate of metal adsorption and this tradeoff has been an obstacle to the solution of the problem.
It is known that a collector that is capable of selective adsorptive recovery of dissolved metals from seawater can be produced by grafting acrylonitrile onto a polyethylene fiber under exposure to radiation and then reacting it with hydroxylamine to introduce amidoxime groups.
It is also known that a selective adsorbent of uranium dissolved in seawater can be produced from a substrate of a desired shape that is made of an inorganic material, an organic material or a composite thereof and into which both an amidoxime group and a hydrophilic group are introduced by radiation-initiated graft polymerization (see Japanese Patent Publication No. 58775/1987) filed by one of present inventors).
Under the circumstances, there has been a pressing need to improve the existing collectors and develop a material that is strong enough to withstand prolonged exposure to hostile weather conditions in ocean and which maintains high performance in collecting vanadium, uranium and other useful metals in seawater.
(b) In seawater, vanadium, uranium and many other rare metals that scarcely occur in Japan are contained dissolved but their concentrations are extremely low, only about 1.9 mg of vanadium per ton of seawater and about 3.3 mg of uranium.
Heretofore, uranium has been recovered from seawater by the following methods using an adsorbent; seawater is brought into contact with the particles of titanic acid to adsorb uranium from the seawater and fine air bubbles are attached to the particles of titanic acid, which are then floated on the seawater and separated therefrom to recover the uranium (Japanese Patent Public Disclosure No. 61018/1979); calcium or carbonate ions are removed from seawater before uranium in the seawater is recovered by adsorption onto a hydrous metal oxide adsorbent (Japanese Patent Public Disclosure No. 79111/1979); a collector produced by reacting a polyethyleneimine derivative with hydroxylamine is used to achieve adsorptive recovery of metal ions dissolved in seawater (Japanese Patent Public Disclosure No. 48725/1987); and using a kalixarene derivative to recover uranium in seawater by adsorption (Japanese Patent Public Disclosure No. 136242/1987).
Dissolved metals can also be recovered using chelate resins and conventional methods based on this approach include the following: a specified group is introduced into a chloromethylated crosslinked polystyrene, which is then reacted with hydroxylamine to produce an adsorbent resin that is used to recover dissolved metals from seawater by adsorption (Japanese Patent Public Disclosure No. 84907/1984); a chelate resin having malonyl dihydroxamate residue is used as an adsorbent to recover dissolved metals by adsorption (Japanese Patent Public Disclosure No. 83730/1984); and a chelate resin having functional groups of a specified structure in the molecule is used to recover dissolved metals by adsorption (Japanese Patent Public Disclosure No. 11224/1985).
To date, the conventional methods of recovering uranium from seawater using adsorbents or those for recovering dissolved metals using chelate resins have not been implemented in practice since they are incapable of cost-effective collection of uranium and other rare metals. However, for Japan which is by no means rich in mineral resources, it has been long desired to exploit the metals that are dissolved in the surrounding sea.